The Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites (TRACERS) Mission Design

Abstract

The detailed study of the global characteristics of collisionless magnetic reconnection that occurs at the magnetopause will be greatly enhanced by observations of plasma fluxes and fields within the low-altitude cusp region, as sampled by two spacecraft orbiting in tandem. The NASA Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites (TRACERS) mission, a Heliophysics Small Explorer (SMEX) mission, will provide the necessary observations to enable significant progress to be made on understanding magnetic reconnection, especially in terms of its temporal versus spatial characteristics. This paper provides an overview of the TRACERS mission design and the trade studies conducted for the optimization of this design.

Keywords: Cusp; Magnetic reconnection; Region of interest; Sun-synchronous orbits; TRACERS.

Fig. 1

Sample northern hemisphere cusp regions (bounded in red), determined from the model described by Eqs. 1–4 and mapped within the science ROI in geomagnetic coordinates, for various IMF and dipole tilt angle values. Local noon (12 MLT) is toward the bottom of each panel. The Low-Latitude Boundary Layer (LLBL) region and the plasma mantle region bounding the high latitude edge are not included in this model cusp. This set of figures illustrates much of the range of variability in the location of the low-altitude cusp as a function of solar wind conditions and Earth’s dipole tilt angle (season)

Fig. 2

Top: Tallies of northern cusp crossings by the TRACERS spacecraft as a function of LTDN for Sun-synchronous orbits, and as a function of prime mission duration (red: 6 months; blue: 1 year; black: 2 years). Bottom: Schematic of three sample Sun-synchronous orbits at 588 km altitude (red: 06:00 LTDN; yellow: 10:30 LTDN; blue: 02:30 LTDN). In addition to the low-altitude cusp location dependences on the solar wind and dipole tilt, the number of anticipated northern cusp crossings by TRACERS is strongly dependent on the orbit configuration during the mission

Fig. 3

TRACERS trajectories for a 10:30 LTDN, 588 km altitude SSO through the northern polar region over the course of a day (15 April), as viewed from above the north Geographic Pole. Each panel illustrates a pass by TRACERS through the science ROI (annular segment). Contours within the ROI indicate the probability of cusp presence at each point along the trajectory

Fig. 4

Schematic of projected IGRF vectors (purple) along an example 10:30 LTDN TRACERS pass through the science ROI. The largest component of each IGRF vector is into the page (i.e. pointing downward into the ionosphere). The orbit Local Aggregate B-field (LABF) (blue arrow) represents the average B-field direction along the trajectory within the ROI. The LABF vector is informative for aligning the TRACERS spacecraft for cusp sampling

Fig. 5

Projections (ECI coordinates) of orbit-by-orbit LABF unit vectors over the course of a single day. Top left: Projections into the XZ_ECI plane. Top middle: Projections into the YZ_ECI plane. Top right: Projections into the XY_ECI plane (vector endpoints only). The largest component of these unit vectors is in the −Z_ECI direction. Bottom: XY_ECI plane projections of orbit-by-orbit LABF unit vectors for four separate days (one for each season) over the course of a year. The changing LABF unit vector is informative for spacecraft pointing operations

Fig. 6

Top panel: Projections into the XY_ECI plane of the orbit-by-orbit LABF unit vectors over the course of a single day (red markers and trace). Example scenario of attitude repointing of the TRACERS spin axis every four orbits (blue markers). Bottom panel: Histogram of the number of cusp crossings as a function of the angle between the spacecraft spin axis and the IGRF at the center of the cusp, for the scenario of repointing every four orbits, over a mission lifetime of two years. The cusp location is determined from the model described in Sect. 3, parameterized by the OMNI solar wind data set advanced in time by one solar cycle (11 years)

Fig. 7

Same as Fig. 6, but for a single spacecraft attitude repointing per day, defined by a daily ‘centroid’ LABF unit vector.

Fig. 8

Comparison of repointing intervals on the alignment of the spacecraft spin axis to the LABF vector within the ROI. Accumulated histograms of the described spacecraft repointing histograms. A conservative once-daily repointing strategy of the TRACERS spacecraft spin axis is sufficient to achieve the TRACERS science goals while significantly simplifying operations

Fig. 9

Left: The angle $\alpha$, between the vector pointing towards the Sun and the LABF unit vector. Right: The variation of $\alpha$ over the course of a year for a 10:30 LTDN Sun-synchronous orbit. This single parameter informs the solar incidence upon satellite surfaces (especially solar panels) during the prime mission

Fig. 10

Left: The angle $\varphi$, representing the deflection of the daily LABF unit vector out of the orbit plane. Right: The variation of $\varphi$ over the course of a year for a 10:30 LTDN Sun-synchronous orbit. This deflection angle is small and varies slowly during the prime mission